The present invention relates generally to articles of footwear, and in particular to articles of footwear having an outsole having improved force distribution and stability on uneven surfaces or terrain.
Most conventional footwear is designed to deflect ground forces by using hard, rigid bottoms. This causes the wearer to absorb much of the force and impact of any uneven terrain underfoot, leading to instability, bruising and risk of injury, including for example, the turning of an ankle. Typical shoe construction, particularly with respect to athletic shoe construction such as sneakers and hiking boots, includes an upper and a sole unit. The sole unit can include multiple layers of material such as foam and/or rubber. Typically these shoes have a hard outsole to withstand abrasion and a soft midsole layer to provide absorption of ground forces. A common problem in footwear, in particular athletic footwear, is that although a softer midsole is desirable for absorption of ground forces, too soft of a midsole allows the heel to displace into the midsole under load conditions. Excessive displacement of the heel often leads to overpronation, causing instability and motion control issues. Such soles typically have a ground contacting portion in the form of an outsole with a number of traction elements thereon, which may project outwardly from the midsole portion of the shoe but, nevertheless, add only minimal force attenuation or cushioning, the bulk of which are dealt with by the midsole of the shoe. In this arrangement, when an object is stepped on or when uneven terrain is encountered, the hard outsole causes the bottom portion of the shoe to react as a unitary structure and often leads to instability when in contact with uneven surfaces. This also reduces the amount of ground contact for the shoe, which can cause traction problems. Furthermore, such a design leads to problematic levels of point loading when objects are encountered. Point loading occurs when force from an object is transferred to the foot of the wearer of the shoe such that the force is concentrated in a small area. Point loading can cause the portion of the sole with which the foot makes contact (typically an insole) to deflect upwardly into the foot, which can cause pain and discomfort for the wearer. This can also cause bruising under foot and can adversely affect whole body stability of the wearer.
Problems with stability and point loading are particularly prevalent in what is generally known as trail running. Trail running is a type of running that differs markedly from road running and track running. Road running and track running often take place on flat or smoothly inclined surfaces. In contrast, trail running generally takes place on hiking trails, most commonly on single track trails, although fire roads are not uncommon. A distinguishing characteristic of such trails is that they are often inaccessible by road except at the trail heads. The trails tend to traverse varying terrain, hills, mountains, deserts, forests, etc. Narrow traverses are common. Likewise, steep inclines or rough terrain sometimes may require hiking or “scrambling.” Runners participating in trail runs must often descend these same steep grades. It is typical for trail runs to ascend and descend thousands of feet. Trail running often takes place in both organized trail races, and as a recreational activity. Common distances in races are 10 km, 20 km, 30 km, marathon (42 km), 50 km, and 50 miles. Anything over marathon distance is considered an Ultramarathon or “Ultra,” which may range up to the 100 mile mark (and beyond). Trail running has become increasingly popular around the world.
With regard to footwear, trail runners have specific and unique needs as opposed to other kinds of runners such as road runners and track runners. By way of example only, road and track runners may prioritize shock attenuation and pronation control in their footwear to deal with terrain like tracks and road surfaces. Trail runners, by contrast, are often challenged to stay upright while running on unpredictable terrain. They are looking for balance of the entire body, not just pronation control. During locomotion, the foot naturally moves through various amounts of what is known in the art as pronation and supination. Such movement is described in general in the article entitled “Standard Test Method For Comparison of Rearfoot Motion Control Properties of Running Shoes,” published by ASTM International in August, 1998, the entirety of which is incorporated by reference herein. Although unique to each individual, many people have an average maximum pronation angle of between 7 and 11 degrees during locomotion on a flat surface. Rear foot angle Θ (as shown in FIG. 31) is generally proportional to pronation angle and is typically on the order of approximately 3 degrees. The amount of pronation and/or supination experienced during trail running tends to vary among a wider range than during road or track running due to the uneven surface of the trails and the frequency with which objects are encountered underfoot, both of which influence the angle of the foot relative to the lower leg. When an object or uneven terrain causes pronation or supination to increase beyond the average maximum range for a given subject, instability begins to occur. If, for example, rear foot angle begins to approach a pronation angle of above about 20 degrees, medial motion of the knee will occur. By further example, if the rear foot angle begins to approach a pronation angle of about 30 degrees, excess motion of the hip can occur, which results in movement of the pelvis. Excess movement of the ankle, knee and hip joints may result in upper body instability. Instability contributes to loss of control or balance. In trail running, there is also a concern regarding protection from bruising under foot which may be caused by repeated impacts with rocks, roots and other irregularities with uneven terrain.
Of additional concern for trail runners is the need for moisture management, lightweight design, in-shoe security that minimizes anterior-posterior movement when running either uphill or downhill and medial-lateral movement while traversing a slope, and traction for a variety of surfaces including irregular surfaces and wet and dry terrain. Current footwear does not adequately address such trail running requirements.
It is therefore desired to provide articles of footwear that can minimize instability and that have soles which can provide the desired force attenuation and traction properties for trail running. These structures may also be beneficial for other forms of conventional running or hiking and in other situations, where objects may also be encountered and inclement conditions can cause problems relating to stability, traction and comfort. Such configurations should further provide lightweight sole designs and should be compatible with various footwear uppers.